Control strategy for filamentous sludge bulking: Bench-scale test and full-scale application.

Sludge bulking caused by the overgrowth of filamentous bacteria, especially Microthrix parvicella, has been observed in WWTPs worldwide during low-temperature periods. In this study, the impacts of sludge load on the in situ growth of M. parvicella and sludge settleability were first evaluated at 15 °C over a period of 500 d using a bench-scale anaerobic-anoxic-aerobic reactor fed with raw sewage from a full-scale WWTP. When the reactor was operated at a sludge load of 0.07 ± 0.015 kg Chemical Oxygen Demand (COD) (kg MLSS·d) for 120 d, the sludge volume index (SVI) increased gradually from 85 mL g to 157 mL g, and the abundance of M. parvicella quantified by qPCR and FISH methods also increased from 0.42% to 4.63% and 1.56%-13.59%, respectively. When the sludge load was further reduced to 0.04 ± 0.004 kg COD (kg MLSS·d), the SVI value varied in a narrow range of 135-164 mL g over a duration of 280 d, while the M. parvicella abundance increased to the maximum values of 10.13% (qPCR) and 18.53% (FISH), respectively. When the sludge load was increased to 0.12 ± 0.016 kg COD (kg MLSS·d), filamentous abundance and SVI were reduced to 1.06% (qPCR) and 105 mL g within 100 d, suggesting that it might be possible to control the growth of M. parvicella by keeping the sludge load above 0.1 kg COD (kg MLSS·d). The feasibility of the strategy was further validated in the same WWTP. It was found that the SVI and filamentous abundance in winter were successfully controlled for two successive years at below 120 mL g and 7% (FISH), respectively, when the sludge load was maintained at 0.14 ± 0.04 kg COD (kg MLSS·d) by adjusting sludge discharge, proving that this sludge-load-based strategy could be an efficient approach to control filamentous bulking.